Snow melt in the Colorado River basin is occurring earlier, reducing runoff and the amount of crucial water available downstream. A new study by NASA/UCLA shows this is due to increased dust caused by human activities in the typically arid American southwest region during the past 150 years.

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The Colorado River is a river in the Southwestern United States and northwestern Mexico which is approximately 1,450 miles long. The natural course of the river flows from the Continental Divide at La Poudre Pass in Colorado, into the Gulf of California between the Baja California peninsula and mainland Mexico.

The watershed of the Colorado River covers 246,000 square miles in parts of seven U.S. states and two Mexican states.

So the Colorado River and how it performs has a major impact on all who live in this area and use the water.

The research team studying changes in the Colorado River was led by Tom Painter, a snow hydrologist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and an adjunct professor in the UCLA Department of Geography. The team examined the impact of human produced dust deposits in mountain snow packs over the Upper Colorado River basin between 1915 and 2003.

Studies of lake sediment cores showed the amount of dust falling in the Rocky Mountains increased by 500 to 600 percent since the mid- to late 1800s, when grazing and agriculture began to disturb fragile but previously stable desert soils.

The team used an advanced hydrology model to simulate the balance of water flowing into and out of the river basin under current dusty conditions and those that existed before the soil was disturbed. Hydrologic data gathered from field studies funded by NASA and others as well as measurements of the absorption of sunlight by dust in snow were combined with the modeling.

More than 80 percent of sunlight falling on fresh snow is typically reflected back into space. In the semi-arid regions of the Colorado Plateau and Great Basin, winds blow desert dust east into the Rocky Mountains where the snow pack is located. Dust will tend to make the snow darker and thereby reducing the snow's ability to reflect sunlight. Dark snow absorbs more sunlight and consequently melts faster and earlier.

Earlier melt seasons change the overall climate conditions down river. Vegetation grows sooner. Plants will lose water to the atmosphere through the exhalation of vapor and an earlier growth season means higher loss of water to the atmosphere than might otherwise occur in this arid region.

The study shows that an annual average of approximately 35 billion cubic feet of water that would otherwise feed the Colorado River is lost from this exhalation and the overall evaporation.

The study showed peak spring runoff now comes three weeks earlier than before the region was settled and soils were disturbed. Annual runoff is lower by more than 5 percent on average, compared with pre-settlement levels.

"The compressed mountain runoff period makes water management more difficult than a slower runoff," Painter said. "With the more rapid runoff under dust accelerated melt, costly errors are more likely to be made when water is released from and captured in Colorado River reservoirs."

Prior to the study, scientists and water managers had a poor understanding of dust-on-snow events. Scientists knew from theory and modeling studies that dust could be changing the way snowfields reflect and absorb sunlight, but no one had measured its full impact on snow melt rates and runoff over the river basin.

Painter believes steps can be taken to reduce the severity of dust-on-snow events in the Colorado River basin. He points to the impact of the Taylor Grazing Act of 1934, which regulated grazing on public lands to improve range land conditions and thereby reduce dusting. Lake sediment studies show the legislation decreased the amount of dust falling in the Rocky Mountains by about one quarter.

"Restoration of desert soils could increase the duration of snow cover, simplifying water management, increasing water supplies and reducing the need for additional reservoir storage of water," Painter said. "Peak runoff under cleaner conditions would then come later in summer, when agricultural and other water demands are greater.

"It could also at least partially mitigate the expected regional impacts of climate change, which include reduced Colorado River flows, increased year-to-year variability in its flow rate and more severe and longer droughts," he added. "Climate models project a 7 to 20 percent reduction in Colorado River basin runoff in this century due to climate change."

For more information: http://www.universityofcalifornia.edu/news/article/24135